In general, construction machinery such as an excavator includes a plurality of actuators for moving the machinery or for driving various work tools and an upper swing body. The plurality of actuators is driven by working fluid discharged from a variable capacity hydraulic pump.
However, there are instances in which the flow discharged from a hydraulic pump exceeds the flow that may be supplied to each actuator when each actuator is stalled or under high load working conditions in a hydraulic system for the above-described constuction machinery. In this case, the surplus flow increases the pressure in the hydraulic system, and when the increased pressure of the working fluid exceeds a relief pressure, the working fluid drains into a tank through a relief valve. Here, the working fluid that drains through the relief valve is of a high pressure that exceeds the relief pressure, and causes a great loss of power in the system.
In particular, because an upper swing body has high inertia, a large portion of the flow of working fluid supplied to the swing motor at the onset of driving the upper swing body is drained into the tank through the swing relief valve, so that the working fluid drained through the swing relief valve causes a large loss of power. In order to reduce such a loss of power, technology is being developed to reduce the flow discharged from a hydraulic pump during swing operation, an example of which is disclosed in Korean Patent Publication No. 2004-0080177.
In a flow control device of a hydraulic pump proposed in the above Korean patent publication, many hydraulic pressure components are needed such as a load pressure sensing passage, a shuttle valve, a pressure intensifier, and a solenoid valve, to sense whether a control valve for a swing motor has been switched, in order to perform controlling to reduce the discharging flow of the hydraulic pump under the relief conditions of the swing motor. Accordingly, when a hydraulic pressure system such as that in the above Korean patent publication is employed, not only is the structure of construction machinery made more complicated, the cost thereof also rises. Also, not only does the pressure loss due to the added hydraulic pressure components cause greater overall loss, but the reliability of the hydraulic pressure system may be diminished.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.